One atom of …….. combines with one atom of bromine.

Options

A. Ar
B. Rb
C. Mg
D. HCI

Explanation

To solve the question "One atom of …….. combines with one atom of bromine," we need to analyze the given options and determine which element can combine with bromine.

### Step-by-Step Solution:

1. **Identify the Options**: The options provided are:
- a) Argon
- b) Rubidium
- c) Magnesium
- d) Hydrogen Chloride (HCl)

2. **Evaluate Each Option**:
- **Option a: Argon**
Argon is a noble gas and is known for its inertness. Noble gases do not readily combine with other elements due to their complete valence electron shell. Therefore, argon cannot combine with bromine.
**Conclusion**: Not a valid choice.

- **Option b: Rubidium**
Rubidium is an alkali metal with a valency of +1. It readily loses one electron to achieve a stable electron configuration. When rubidium combines with bromine (which has a valency of -1), they form rubidium bromide (RbBr). Thus, one atom of rubidium can indeed combine with one atom of bromine.
**Conclusion**: This is a valid choice.

- **Option c: Magnesium**
Magnesium is an alkaline earth metal with a valency of +2. It typically combines with two bromine atoms to form magnesium bromide (MgBr₂). Since we are looking for a combination with only one atom of bromine, magnesium does not fit this criterion.
**Conclusion**: Not a valid choice.

- **Option d: Hydrogen Chloride (HCl)**
HCl is a compound and does not represent a single atom. Additionally, HCl does not react with bromine because chlorine is a stronger oxidizing agent than bromine. Therefore, no reaction occurs between HCl and bromine.
**Conclusion**: Not a valid choice.

3. **Final Decision**: After evaluating all options, we find that the only element that combines with one atom of bromine is rubidium. Therefore, the answer is:
- **Rubidium**.

### Final Answer:
One atom of **Rubidium** combines with one atom of bromine.

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